1. Field of the Invention
The present invention relates to a method for forming a back-side illuminated image sensor comprising pixels isolated from one another.
2. Discussion of the Related Art
Back-side illuminated image sensors are increasingly used in photodetection devices. In such sensors, conductive tracks and vias connecting the different photodetection components are formed on a first surface of a semiconductor layer, the device being illuminated on the second surface or back side of this layer. Thus, the conductive tracks and vias do not disturb the passing of light rays intended to photogenerate electron/hole pairs in the semiconductor layer.
A back-side illuminated image sensor is generally constructed as follows. It is started from a structure of semiconductor-on-insulator type (SOI), that is, a single-crystal semiconductor layer resting on a semiconductor support or substrate with an interposed oxide layer. Regions for trapping photogenerated carriers and elements capable of transferring these carriers, for example, MOS transistors, are formed inside and on top of the semiconductor layer, these elements defining pixels. A stack of interconnection levels in which are formed conductive tracks and vias separated by an insulating material, is formed on the device. The conductive tracks and vias enable to connect the different electronic elements of the device together and to external contacts.
Then, another substrate, generally a silicon wafer called handle wafer, is attached to the upper portion of the interconnect stack. The initial semiconductor support or substrate, supporting the semiconductor layer, is then removed by using the oxide layer of the SOI structure as a stop layer. Then, an antireflection layer, color filters, and/or microlenses are formed on the apparent surface or back side of the semiconductor layer intended to be illuminated, opposite to the interconnect stack.
A permanent concern in the forming of back-side illuminated image sensors is the isolation between pixels of a same sensor. Indeed, crosstalk phenomena may occur between two pixels, be it by traveling of a photon towards a neighboring pixel (optical crosstalk) or by traveling of a photogenerated carrier (electron or hole) towards a neighboring pixel (electronic crosstalk). Such parasitic phenomena result in erroneous detections.
Thus, there is a need for a method for forming a back-side illuminated image sensor which strongly limits such parasitic effects.